1. Field of the Invention
The present invention relates to data modulation and demodulation in a communication network, and, more particularly, to space-time coding and decoding techniques for wideband data channels.
2. Description of the Related Art
Wireless channels exhibit a number of impairments, among which fading is one of the most severe. For narrowband channels, the fading can often be assumed to be flat, while for wideband channels the fading is typically frequency selective. In addition, additive noise and interference contribute significantly to signal degradation. Diversity is a method to improve transmission over fading channels. Time diversity uses encoding to duplicate and spread information through an encoded bit stream (e.g., convolutional encoding) and space diversity employs multiple transmit and/or receive links to duplicate and spread information over multiple signal paths.
Coded modulation systems employ methods that utilize time diversity. Encoded data is transmitted through the path between a single transmit antenna and a single receive antenna. Some methods efficiently utilize binary convolutional codes to obtain diversity gains with higher-order, non-binary modulation symbols (e.g., 16-QAM), such as bit-interleaved coded modulation (BICM) systems using multi-level coding methods (and corresponding multistage decoding at the receiver). For example, BICM systems provide diversity gains, and, for example, higher-order, coded modulation systems use well-known binary convolutional codes separated by interleaving to encode the data. Further improvements in system performance are obtained by iterative demapping (translation of symbols to bits) and decoding at the receiver. More recently, so-called space-time coding methods have been proposed to obtain both space and time diversity by using multiple transmit and/or receive antennas along with matching coding. For example, a space-time BICM scheme for narrowband radio channels employing multiple transmit antennas in flat fading cases is described in A. M. Tonello, “Space-Time Bit-Interleaved Coded Modulation With An Iterative Decoding Strategy,” Proceedings, VTC 2000 Fall, Boston, Mass., September 2000, pp. 473-478, which is incorporated herein in its entirety by reference.
Orthogonal Frequency Division Multiplexing (OFDM) is a form of data transmission in which a block of data is converted into a parallel form and mapped into frequency domain symbols. To generate a time domain signal for transmission over the antenna link between antennas, the inverse discrete Fourier transform (IDFT, or its fast version, the IFFT) of length F is applied to F frequency domain symbols to create F subchannels (also known as F subcarriers, since each channel is a separately modulated carrier). Each of the F subcarriers is orthogonal to each other while the frequency spectrum overlaps. The frequency spacing between the F subcarriers is minimum in OFDM, giving OFDM high spectral efficiency. At the receiver, the discrete Fourier transform (DFT, or its fast version, the FFT) is applied to the received signal over F subchannels to generate a sequence of values representing estimated frequency domain symbols. Demapping maps the estimated symbols back to the original block of user data (bits). OFDM allows for wideband transmission over frequency selective (radio) channels without adaptive equalizers. For wideband systems, OFDM has been proposed for a wide range of radio channel applications. One application is the wireless Local Area Network (LAN) system defined by the IEEE 802.11a standard. This standard adopts OFDM in packet-based communications operating in unlicensed 5-GHz bands.